I have had no end of problems getting block element alignment successfully under control in all browsers. Most reliable:IE (though probably not stds-compliant), most stubborn:Opera (which probably is).
By making the left offset exactly -0.5 of the width, the element will center-align.
By playing with the values of width and left you should be able to achieve the same effect you want reliably in all major browsers.
Hope this helps
By the way, if the rendering of your page is correct in IE6 (which I am currently using) then table spacing/alignment is not the only thing to look at in Firefox, where the header renders very differently.